Gantry cranes are commonly used in ports, rail yards or other intermodal shipping facilities for lifting and moving objects such as containers and truck trailers. Such cranes are equipped with various grappler mechanisms to accommodate certain container configurations and associated standard latching systems. For example, highway trailers are typically lifted with a grappler having a swing-arm mechanism, and a standard shipping container typically has four twistlock latches located at the upper four corners of the container for lifting with a grappler having a plurality of corresponding twistlocks. Some grapplers are equipped with both swing arms and twistlocks for selective use as appropriate.
To lift a trailer, shipping container or the like, the crane operator typically maneuvers the crane into a position wherein the crane straddles the object to be lifted. The operator then adjusts the position of the grappler so as to bring the grappler into engagement with the object. To this end, the crane is configured so that the grappler can move in both a side-to-side or transverse direction and a vertical direction.
Unfortunately, due to operator misjudgment of the position of the grappler or other errors, the trailers and containers are occasionally damaged when the operator tries to engage the trailer or container with the grappler prior to lifting. For example, the roof of the object to be lifted can be damaged if the operator does not properly position the grappler or moves the grappler at too high a speed when it is lowered into engagement with the object. In addition, there is a risk that the object could be dropped and damaged if the operator does not properly engage the grappler with the object.